This works if you enable NoMonomorphismRestriction. Cheers, Adam On Thu, 22 Feb 2018 at 23:41 Tom Ellis < tom-lists-haskell-cafe-2013@jaguarpaw.co.uk> wrote:
I'm puzzled by GHC's behaviour in the following program.
'baz = bar . foo' does not work because there is "no instance for ...". But if I manually assume those instances in the context all is fine. Why can GHC not infer that context? Is there any extension or clever trick I can use to get this to infer like I want?
Thanks,
Tom
{-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE FlexibleContexts #-}
class Foo a b class Bar a b type family Quux a
foo :: ( Foo a b , b ~ Quux a ) => a -> Quux a foo = undefined
bar :: Bar a b => a -> b bar = undefined
-- Doesn't work -- No instance for (Bar (Quux a0) c0) arising from a use of ‘bar’ -- No instance for (Foo a0 (Quux a0)) arising from a use of ‘foo’ --baz = bar . foo
baz' :: ( Foo a (Quux a) , Bar (Quux a) b ) => a -> b baz' = bar . foo _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.